Silicon carbide (SiC) is rarely found in nature. It has, however, been manufactured for more than eighty years, in crystalline form, for abrasive products. Silicon carbide crystals found in nature and in abrasive products are generally black and not translucent because they contain substantial levels of impurity atoms.
Because of the theoretically quite favorable electronic properties of silicon carbide, significant development activities were initiated during the 1960s and 1970s with the objective of growing large (bulk) crystals of low impurity silicon carbide for use in the production of semiconductor devices. These efforts finally resulted in the commercial availability of relatively low impurity, translucent silicon carbide crystals. These silicon carbide crystals are fabricated and marketed as very thin, green, amber or blue (175 .mu.m-400 .mu.m) slices useful for semiconductor devices.
Recently, it has been discovered that relatively low impurity, translucent, single crystal silicon carbide may be grown with a desired color and thereafter cut and fashioned into synthetic gemstones. These gemstones have extraordinary hardness, toughness, chemical and thermal stability, and a high refractive index that produces unparalleled brilliance. The single crystals from which the gemstones are produced have been grown by sublimation according to techniques of the type described in U.S. Pat. No. Re. 34,061.
Silicon carbide crystals can be grown in a wide range of colors (including green, blue, red, purple. yellow, amber and black) and shades within each color by the appropriate selection of dopants (e.g., nitrogen and aluminum) and by varying the net doping densities (concentrations). Because of its wide bandgap, undoped ("intrinsic") silicon carbide crystals in the hexagonal or rhombohedral forms are inherently colorless. Thus, silicon carbide crystals offer the potential to be cut and fashioned into gemstones of many various appearances, including that of diamond.
Although the colored crystals have proven relatively easy to produce, problems have been encountered in creating the exceedingly impurity-free sublimation system environment necessary for growing undoped, colorless silicon carbide. Because colorless silicon carbide gemstones have an extraordinary appeal, there is a need for a more cost effective and reliable process for growing large single crystals of silicon carbide in colorless form.